Kinetics of oxidation of hydrazine by a dioxo-bridged dimanganese(III,IV) complex ion

In aqueous acidic media containing an excess of Hbipy(+)-bipy buffer in the pH 3.5-4.5 range, the complex ion [(bipy)(2)Mn-III(mu -O)(2)Mn-IV(bipy)(2) ](3+) (1) coexists in rapid equilibrium with its diaqua derivative [Mn-2(II I,IV) (mu -O)(2)(bipy)(3)(H2O)(2)](3+) (1a) (bipy = 2,2'-bipyridine). An ex cess of N2H5+ quantitatively reduces the mixture to Mn-II, itself being oxi dised to N-2. The first order rate constant, k(o) decreases with increasing C-bipy (C-bipy = [Hbipy(+)] + [bipy]) but increases with increasing [N2H5] and [H+]. The observed kinetic dependence can be explained in terms of a reaction between (1a) and N2H5+. Replacement of solvent H2O with D2O decrea ses k(o) substantially and the effect suggests simultaneous transfer of an electron and a proton in the rate-determining step. The relevance of this o bservation to the delayed oxidation of H2O in the hydrazine-treated photosy stem II is discussed.